A printed circuit board perforating laser machining apparatus as an example of a laser apparatus having a function of scanning with laser beams is an apparatus for irradiating a printed circuit board with pulsed laser beams so as to make holes for connecting conductor layers of the board with each other. For example, a background-art printed circuit board perforating laser machining apparatus has an XY table servo mechanism and a pair of optical scanner servo mechanisms (for example, see JP-A-2002-137074, page 2 and FIG. 6). A printed circuit board is mounted on the XY table servo mechanism and moved thereby in X)- and Y-directions within a horizontal plane. The pair of optical scanner servo mechanisms are provided for scanning the printed circuit board with laser beams in the X- and Y-directions.
As for the structure of an optical scanner, for example, a coil is fixed to a central section of a single-piece penetrating rocking shaft, and a pair of bearings are disposed adjacently to the opposite ends of the coil. An angle detector is disposed outside one of the bearings, while a mirror mounting portion is disposed outside the other bearing (for example, see JP-A-2002-6255, page 2 and FIG. 6).
Description will be made below more in detail with reference to the drawings.
FIG. 5 is a block diagram showing an example of the configuration of a mirror servo mechanism (optical scanner servo mechanism) in a background-art laser machining apparatus.
An electromagnetic rocking actuator 110 of a optical scanner 100 rocks a rocking shaft 111. A mirror 130 is attached to one end portion of the rocking shaft 111 with a mirror mount 131, while an angle detector 120 is attached to the other end portion.
Due to the aforementioned configuration, the direction of the mirror 130 is changed in accordance with the rocking of the rocking shaft 111 so that the outgoing direction of a laser beam 30 incident on the mirror 130 is changed. The rocking angle of the rocking shaft 111, that is, the mirror 130 is detected by the angle detector 120.
An upper control unit 10 compiles an NC program and gives an command to a optical scanner control unit 20 as to a target positioning angle 11 of the mirror 130 in accordance with the position on a to-be-machined piece on which the laser beam 30 should be positioned.
FIG. 6 is a block diagram of a scanner servo mechanism constituting the background-art optical scanner control unit 20. A portion to be executed by software with a servo processor is illustrated on the left of the broken line, and the connection relationship of hardware and the flow of signals are illustrated on the right.
A target trajectory generating unit 210 calculates a target value 215 of the rocking angle of the optical scanner every moment based on the target positioning angle 11, and generates a target trajectory of the scanner servo mechanism. A detected rocking angle 255 of the scanner is subtracted from the target value 215 by a subtracting unit 222 so as to obtain a deviation 225. The deviation 225 is subjected to control and processing in a compensating element 220 so that a manipulated variable signal 226 is calculated. The manipulated variable signal 226 is converted into an analog signal in a D/A converter 230. Thus, an commanded value (driving signal) 21 of a current control system 240 is obtained. An armature of the actuator 110 is connected to the output side of the current control system 240, and a current detection resistor 241 is connected in series with the armature. The terminal voltage of the current detection resistor 241 is detected by a differential amplifier 242, and fed back to the current control system 240 as a current signal. An encoder 120 linked with the rocking shaft 111 generates pulses (position signal) 22 in accordance with the rocking quantity. The pulses are counted in a pulse counter 250 and fed back as a rocking angle 255.
When the aforementioned processes are repeated, the mirror 130 approaches the target position gradually. When the mirror 130 has been positioned, a positioning completion signal 12 is sent from the optical scanner control unit 20 to the upper control unit 10.
The laser beam 30 outputted from a not-shown laser oscillator is reflected by the mirror 130. Thus, a machining position of the piece to be machined is irradiated with the reflected laser beam 30 through an Fθ lens 140. In FIG. 5, three machining positions A, B and C corresponding to three rocking angles of the mirror 130 are illustrated.
FIG. 7 is a sectional view of the actuator.
A cylindrical inner yoke 112 is disposed to surround the rocking shaft 111. Outside the inner yoke 112, circumferentially divided four permanent magnets 113a, 113b, 113c and 113d are disposed to be separated from the inner yoke 112 through a cylindrical air gap G. The permanent magnets 113a to 113d have been magnetized to be polarized radially. The permanent magnets 113a and 113c are magnetized in one direction, while the permanent magnets 113b and 113d are magnetized in the opposite direction.
An outer yoke 114 is disposed outside the permanent magnets 113a to 113d, and these parts form a magnetic circuit. Due to a magnetic field formed by the permanent magnets 113a to 113d and the inner yoke 112, a magnetic flux M is generated substantially radially in the air gap G. In addition, strand sets 115a, 115b, 115c and 115d forming the coil of the armature are disposed in the air gap G as illustrated.
With the configuration described above, when a current is applied to the coil, a current flows in the illustrated direction in the strand sets 115a, 115b, 115c and 115d. Due to the interaction between the magnetic flux and the current, a force (Lorentz force) acts on the strand sets 115a to 115d circumferentially. Since the coil, that is, the strand sets 115a to 115d are fixed to the rocking shaft 111, the force serves as a torque for driving the rocking shaft 111. The torque is proportional to the current flowing through the coil, and the proportionality factor is a torque constant.
In recent years, there has increased a requirement to improve the efficiency in machining with a optical scanner application product such as a laser machining apparatus. High speed response is also required to a optical scanner servo mechanism. A optical scanner operates to rock a mirror within a limited angle range, but the characteristic of the optical scanner is not always uniform in this angle range.
That is, in the optical actuator 110 shown in FIG. 7, the magnetic flux M near the center of each permanent magnet 113a-113d trends radially, but tilts with respect to the radial direction and is low in density as approaches an end portion of the permanent magnet 113a-113d. Therefore, the torque constant is lowered when the strand sets 115a-115d approach the end portions of the permanent magnets 113a-113d. 
FIG. 8 is a graph showing a relationship between the rocking angle and the torque constant. The rocking angle θB designates the center of rocking, the rocking angle θA designates a negative-side angle, and the rocking angle θC designates a positive-side angle. The rocking angles θA, θB and θC correspond to the machining positions A, B and C in FIG. 5 respectively.
FIG. 9 is a graph showing deviation signals of the servo mechanism when positioning was performed in identical strokes in the machining positions with the rocking angles θA, θB and θC.
As is apparent from FIG. 9, a suitable response is shown at the rocking angle θB, but overshoot appears at the rocking angle θA or θC. That is, due to the torque constant changing in accordance with the rocking angle, there occurs a problem that the servo mechanism having good positioning responsiveness at one angle produces overshoot or overdamp at another angle.
However, it is difficult to uniformalize the intensity of the magnetic field acting on the coil independently of the rocking angle. Accordingly, the magnitude of the torque acting on the rotor alters in accordance with the rocking angle in spite of the same current applied thereto. In the same manner, even in an optical scanner generating torque by use of another structure, it is difficult to prevent the torque constant from having any alteration depending on the rocking angle.